A composite video signal contains information which is used by a video system to generate a video picture on a display, monitor or television. Each period, within the horizontal portion of a composite video signal contains information representing one horizontal output line which is to be output on the video display, monitor or television. Each horizontal period includes a horizontal synchronization pulse, a burst signal and a video information signal. In many video transmission systems, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. The horizontal synchronization pulse is used by a phase locked loop to synchronize the system for displaying the next horizontal line of video information. The burst signal is used to synchronize the phase of the sampling pulses with the phase of the color subcarrier signal. Separator circuits are utilized to separate the horizontal synchronizing signal and the burst signal from the incoming video signal. The burst signal consists of a sinusoid with a frequency equal to 3.58 MHz, which is the frequency of the chrominance subcarrier f.sub.SC. The video information signal then comprises the chrominance subcarrier having different phases amplitude-modulated with chrominance information. The composite color video signal includes both luminance and chrominance information.
A video picture or frame is made up of a number of horizontal lines included within the video display. To display a video picture or frame the video system begins at the top of the screen and displays the information within the composite video signal one horizontal line at a time. The information for each horizontal line is contained within a horizontal period of the composite video signal. After each horizontal period, the video system moves to the next line and displays the information within the next horizontal period of the composite video system. This process continues until the video system reaches the bottom line on the video display.
After displaying the video information on the bottom line of the video display, a conventional video system resets itself to the top of the display in order to begin displaying the next frame. In order to allow the system to reset itself to the top of the video display a vertical blanking period is included within the composite video signal after the video information for each frame. This vertical blanking period allows the video system sufficient time to reset to the top of the video display and begin displaying the information for the horizontal lines of the next frame. Therefore, a number of horizontal periods, enough to comprise a frame or screen, are strung together, within the composite video signal. The composite video signal includes a vertical blanking period between each frame which allows the video system to perform a vertical reset and prepare to display the next frame by moving back up to the top of the video display.
During the vertical blanking period the composite video signal includes a first period of equalizing pulses, a period of serration pulses and a second period of equalizing pulses. During this vertical blanking period the video system resets itself to the top of the video display so that it is ready to begin displaying the information for the next frame. However, the video system must be notified of or be able to detect the vertical blanking period so that it can reset itself to the top of the video display. The serration pulses carry synchronization information used by the local vertical oscillator, within the video system, during a vertical reset.
The horizontal synchronizing pulses and the vertical synchronizing pulses are combined together into a composite synchronizing signal CSYNC. A device receiving this composite synchronizing signal then extracts the horizontal synchronizing pulses and the vertical synchronizing pulses from the composite signal. The equalizing and serration pulses are all generated during the vertical blanking period at a frequency equal to twice the frequency of the horizontal synchronizing pulses.
A sync separator circuit 10, as illustrated in FIG. 1, is used to separate all of the synchronization pulses from the composite video signal including the horizontal, equalizing and serration pulses. However, the sync separator circuit separates the synchronization pulses by comparing their amplitude with respect to the blank level of the signal and therefore has no way of differentiating between horizontal synchronization pulses, equalizing pulses and serration pulses. The output of the sync separator circuit is used by a horizontal phase-locked loop to lock the video system in phase with the composite video signal during the horizontal period of each frame. During the vertical blanking period, the sync separator circuit is configured to output the equalizing and serration pulses which are generated at twice the frequency of the horizontal synchronization pulses. Thus, twice as many synchronization pulses are generated during the vertical blanking period as during the horizontal period. The horizontal phase-locked loop will therefore be unable to remain locked during this period unless something is done to alter the frequency of synchronization pulses during the vertical blanking period.
Conventional circuits use precision timing signals from voltage ramps to provide a mask for the equalizing pulses. Such circuits are typically referred to as Half H Killer (HHK) circuits because the extra pulses which are removed are included halfway between adjacent horizontal synchronization pulses. The voltage ramp signals used by HHK circuits are generated by storing charge on a capacitor. Typically, the period of these ramps is relatively long, up to 64 microseconds. Accordingly, either an extremely small current or a very large capacitor are required to efficiently support a period of that length. When an extremely small current is used, small base current variations in the transistors cause large percentage differences in the small reference current, affecting the precise timing nature of the ramp circuit. External components are conventionally used to generate the necessary current and threshold voltage signals because they may be selected for high absolute accuracy. Such, external components are undesirable because they add costs to a system, take up extra space on a printed circuit board within the system and require a dedicated pin on an integrated circuit to which they are coupled. What is needed is an internal current source and a threshold voltage generation circuit capable of generating a small precise current and corresponding threshold voltage signal without the use of external components.